A wheel-support rolling bearing unit for rotatably supporting an automobile wheel with respect to the suspension device comprises; an inner member (for example, rotating ring, hub ring) having an inner raceway on an outer peripheral surface, an outer member (for example, stationary ring) having an outer raceway on an inner peripheral surface, and a plurality of rolling elements rotatably provided between the inner raceway and the outer raceway. Moreover, many wheel-support rolling bearing units having complex shapes and provided with a flange for fastening the wheel or a part of the suspension device to the peripheral surface of the inner member or outer member, have been conventionally used. Furthermore, holes for passing studs or bolts for fastening the wheel or a part of the suspension device, are formed in a plurality of places around the circumferential direction of the flange.
On the other hand, when the rolling bearing is in use, high surface pressure is repeatedly loaded from the respective rolling elements to the outer raceway and the inner raceway. Accordingly, in order to ensure the rolling fatigue life of the rolling bearing, it is necessary to increase the surface hardness and the wear resistance of the outer raceway and the inner raceway. Considering such a situation, conventionally, for the standard rolling bearing, a material made of high carbon chrome steel such as SUJ2, which is quenched and tempered to harden the whole surface, has been used.
However, of the inner member and the outer member constituting the wheel-support rolling bearing unit, a member provided with a flange has a complex shape compared to the inner member and the outer member constituting the standard rolling bearing. Accordingly, considering to ensure the hot forging property, the cutting property, and the drilling property, the member provided with the flange which is made from a medium carbon steel such as S53C wherein a hardened layer is formed by induction hardening on the inner raceway or the outer raceway and on the periphery of the raceway portions, has been used in many cases.
For example, a wheel-support rolling bearing unit 1 as shown in FIGS. 1 and 2, for rotatably support an automobile wheel with respect to a suspension device, has been widely used.
The wheel-support rolling bearing unit 1 shown in FIG. 1 comprises a hub ring 2 and an inner ring 3 constituting the rotating ring (inner member), an outer ring 4 being the stationary ring (outer member), and a plurality of rolling elements 5. A flange 6 for supporting the wheel is formed at the outside end portion of the outer peripheral surface of the hub ring 2 (outside in the widthwise direction means the side towards the widthwise outside when assembled in the vehicle; the left side in FIGS. 1 and 2, while conversely, the side towards the widthwise center is the inside in the widthwise direction; the right side in FIGS. 1 and 2). Moreover a first inner raceway 7a is formed on the outer peripheral surface of a middle portion of this hub ring 2. Similarly, a step portion 8 having a reduced outer diameter is formed at the inside end portion thereof. Furthermore, the inner ring 3 having a second inner raceway 7b formed on the outer peripheral surface is fitted onto the step portion 8, thereby constituting the rotating ring. Moreover, the inner end face of this inner ring 3 is pressed by a crimped portion 9 which is formed by swaging radially outwards a cylindrical portion formed at the inside end portion of the hub ring 2, so that the inner ring 3 is fixed in place on the hub ring 2. A flange 11 for a suspension device is provided on the outer ring 4. Double row outer raceways 10a and 10b are formed on the inner peripheral surface of the outer ring 4. A plurality of rolling elements 5 are rotatably provided respectively between the outer raceways 10a and 10b and the inner raceways 7a and 7b. 
Next, in the wheel-support rolling bearing unit 1 shown in FIG. 2, a hub ring 2 being the rotating ring with a flange 6 for supporting the wheel formed on the outer peripheral surface, is arranged around a pair of inner rings 3 being the stationary rings which are fitted onto a supporting shaft (not shown) and are not rotatable. A plurality of rolling elements 5 are provided respectively between the outer raceways 10a and 10b which are formed on the inner peripheral surface of this hub ring 2, and the inner raceways 7a and 7b which are formed on the outer peripheral surface of the respective inner rings 3.
In either of the examples shown in FIGS. 1 and 2, balls are used as the rolling elements 5. However in some cases, taper rollers may be used as the rolling elements in the case of a rolling bearing unit for an automobile which is of increased weight.
In order to assemble the abovementioned wheel-support rolling bearing unit 1 to an automobile, if the structure is such as in FIG. 1, the attaching portion 11 in the shape of an outward flange formed on the outer peripheral surface of the outer ring 4, is screw fastened to a component of the suspension device such as the knuckle, so that the outer ring 4 being the stationary ring, is supported on the suspension device. If the structure is such as in FIG. 2, the pair of the inner rings 3 are fixed onto the supporting shaft so that the respective inner rings 3 being the stationary ring, are supported on the suspension device. In any case, the wheel is fixed to the flange 6 which is formed on the outer peripheral surface of the hub ring 2 (inner member in FIG. 1 and outer member in FIG. 2). As a result, the wheel can be rotatably supported with respect to the suspension device.
The hub ring 2 constituting the wheel-support rolling bearing unit 1 as described above is made from a medium carbon steel such as a carbon steel for machine structural use such as S53C, considering to ensure the hot forging property and the cutting property. During production, firstly a bar-shaped material cut into a predetermined length is heated to the austenite range at around 1100 to 1200° C. by high frequency induction heating. Then it is formed into a predetermined shape by hot forging, and cooling is performed. During this process, a complex structure of pro-eutectoid ferrite and pearlite can be obtained by pearlite transformation which occurs during the time since the pro-eutectoid ferrite is precipitated from the austenite grain boundary until it is cooled to room temperature. Heat treatment such as quenching and tempering is not applied to most parts of such a structure, it being used as is.
On the other hand, if the structure is such as shown in FIG. 1, a hardened layer is formed by induction hardening in the region from the root portion on the inside surface side in the axial direction of the flange 6 and the first inner raceway 7a to the step portion 8 as shown by hatching in FIG. 1, in order to ensure the rolling fatigue life and to prevent fretting at the fitting portion.
In the case of such a structure, a portion of the members having the flange 6, which is not subjected to induction hardening, is used in the condition where only the hot forging is applied (hardening treatment is not applied). In the description hereunder, non-thermal refined portion denotes a portion which is not subjected to induction hardening and is used in the condition where only hot forging has been applied.
On the other hand, recently, in order to increase the fuel efficiency and the running performance of automobiles, the wheel-support rolling bearing unit 1 has been required to be lightened, and it has also been considered to thin the wall of the flange 6 for supporting the wheel. However, if the wall of the flange 6 is thinned, since the strength of the root portion of this flange 6 is weakened, sufficient consideration is required so as to ensure the strength when thinning the wall.
Particularly at the root portion on the outside surface side of the flange 6, bending stress is concentrated due to the moment load applied to the wheel-support rolling bearing unit 1 between the suspension device and the wheel, at the time of turning travel and the like. Accordingly, if no countermeasures are taken, damage such as cracks are possibly produced based on the metal fatigue. On the other hand, as described above, the root portion on the inside surface side of the flange 6 is very strong since a hardened layer is formed thereon by induction hardening, so that the fatigue strength is higher than that of the root portion on the outside surface side, and damage such as cracks are less likely to occur.
Moreover, since a torsional stress is applied to the flange 6 accompanied with the rotation of the wheel, if this flange is the non-thermal refined portion, damage such as cracks is easily produced also in this flange 6. Therefore, in order to thin the wall of this flange 6, it is necessary to increase the rotating bending fatigue strength and the torsional fatigue strength of the non-thermal refined portion in this flange and the root portion of this flange.
On the other hand, in Japanese Unexamined Patent Publication No. 2002-87008, a structure is described where the strength of the root portion on the outside surface side of the flange is increased by forming a surface hardened layer also on the root portion on the outside surface side of the flange by induction hardening, similarly to the root portion on the inside surface side of the flange.
Moreover, in Japanese Unexamined Patent Publication No. 2001-200314, it is described that, with the object of ensuring workability of the member which is rotating together with the wheel at the time of usage (rotating member), and increasing the rolling fatigue life of the raceway having the hardened layer, this rotating member is made from an alloy steel wherein the C content is more than S53C and less than SUJ2, and alloy components such as Si, Cr and the like are added.
However, in the bearing unit for a wheel described in Japanese Unexamined Patent Publication No. 2002-80778, the cost is increased due to an increase in the induction hardened portion at the outside root portion 14, and there is concern of a decrease in the shock-proof due to quenching and hardening the inside root portion 12 and the outside root portion 14 of the flange 6 for attaching a wheel.
Moreover, in the conventional structure described in Japanese Unexamined Patent Publication No. 2001-200314, there is no consideration given to increasing the rotating bending fatigue strength or the torsional fatigue strength of the non-thermal refined portion.
Therefore, in order to proceed to thin the wall of the flange 6 for attaching a wheel without quenching and hardening the outside root portion 14 of the flange 6, it is necessary to increase the endurance ratio (fatigue limit strength/tensile strength), considering the fatigue strength and the cutting property after forging.
Moreover, recently, in the wheel-support flanged bearing unit, with an object of suppressing vibration during running and partial wear of a brake, highly accurate working of the brake rotor fastening surface of the flange 6 has been required. Since the flange 6 is worked by turning and drilling, a good cutting property and drilling property of the material has been further strongly required. However, in the flanged bearing unit described in the abovementioned Japanese Unexamined Patent Publication No. 2002-200314, problems with the cutting property and the drilling property of material have not been solved. If the cutting property and the drilling property are poor, the productivity and the tool life are decreased, being a factor for increasing the cost.
Furthermore, as a method of increasing the cutting property and the drilling property of the material, it is effective to decrease the C amount contained in the steel. However, if the C amount is decreased, there is a problem of decreasing the rolling fatigue life of the induction hardened raceway 13.
In addition, since the rotating ring (wheel) side rotates while supporting the load, repetitive rotating bending stress is generated in the root portion of the flange 6. Since the root portion of the flange 6 includes the non-thermal refined portion which is not subjected to induction hardening, fatigue strength of the non-thermal refined portion is also required.
The present invention takes the above problems into consideration with an object of providing a flanged bearing unit which increases the fatigue strength of the flange without increasing the induction hardened portion, so as to enable lightening of the flange by thinning the wall, and a method of manufacturing the bearing unit.
Moreover, as described above, the hub ring 2 constituting the wheel-support rolling bearing unit 1 is made from a medium carbon steel such as S53C, in consideration of ensuring the hot forging property and the cutting property. During production, the bar-shaped material cut into the predetermined length is heated by high frequency induction. Then, hot forging is applied in the austenite range at around 1100 to 1200° C. so as to form the product. Most parts are used without having quenching and tempering applied. However, in the region from the inside root portion 12 of the flange 6 through the inner raceway surface 7a to the small diameter step portion 8, an induction hardened layer 13 (shown by hatching in FIG. 1) is formed with the object of ensuring rolling fatigue life and preventing fretting of the inner ring fitting portion. The portions which are not subjected to induction hardening are used in the condition of heat treatment where only the hot forging has been applied (non-thermal refined portion).
At this time, firstly, due to the cooling after hot forging, a ferrite structure is produced and the rest becomes a pearlite structure. In this manner, since the two phases have different transformation temperatures, then due to the effect of the cooling rate in the vicinity of the transformation point and the size of the austenite grains which are grown at the time of heating (these may also be called prior austenite grains due to the relation of ferrite-pearlite), the transformation behavior is changed and a structure having a different final ferrite-pearlite fraction can be obtained.
At the time of assembling the wheel-support rolling bearing unit 1, after fitting the inner ring 3 onto the small diameter step portion 8 of the hub ring 2, as shown in FIG. 1, there is a step for flaring the cylindrical portion 9 of the hub ring 2 radially outwards in order to fix the inner ring 3 and the hub ring 2. In this step, cold working with a high processing rate is performed at a high rate of strain, wherein a load oriented radially outwards is applied to the cylindrical portion 9 while rotating the whole wheel-support rolling bearing unit 1.
However, when such swaging is performed, there has been a problem of cracking in the radial direction on the surface. Therefore, in Japanese Unexamined Patent Publication No. 2002-139060, a technique for suppressing the cracks by rounding the corner of the end surface of the cylindrical portion, is proposed. In Japanese Unexamined Patent Publication No. Hei 12-087978, a technique for increasing slip-off resistance by improving the curved shape of the final surface crimped from the cylindrical portion is proposed.
However, the abovementioned problems can not be solved in some cases if the deformation load and the deformation velocity when swaging is increased. Here, an object of the present invention is to solve the above problems and to provide a wheel-support rolling bearing unit wherein cracks are unlikely to occur at the time of swaging.